Cordless telephone (CT) systems, such as the digital cordless telephone system CT2, have recently been introduced into the market. Such systems comprise a base station intended to be connected to a public switched telephone network (PSTN). There are three different types of base stations: a residential (home) base station, a base station connected to a private branch exchange (PBX) in an office or configured to operate as a PBX, and telepoint base stations, by means of which outward calls only are possible for a CT telephone.
As the present-day CT systems are regarded as PSTN subscriber connections (access points in PSTN), a call is addressed to the CT terminal equipment in a normal way by dialing a telephone number in accordance with the PSTN numbering scheme. On the basis of the telephone number the PSTN thereby routes the call to a respective subscriber connection to which the CT residential base station or the PBX of the CT office base station is connected. The base station pages the CT terminal equipment over the radio path by means of an identity code determined by the numbering scheme of the CT system, and sets up a call when the terminal equipment responds. In practice, it is thus possible to make a call to the CT terminal equipment only through its residential or office base stations (through predetermined subscriber connections).
Accordingly, the numbering schemes of the present CT systems themselves serve only the CT radio link and are not utilized anywhere in the PSTN.
In the future it may become advantageous to be able to connect CT systems even to mobile radio networks. As there are no addressable access points (such as PSTN subscriber connections) but only common traffic channels shared by all mobile subscriber equipments in the mobile radio network, the setup of a call is always based on the use of an identity code assigned to the subscriber equipment.
A problem therewith is, however, that the numbering schemes specified for the cordless telephone system and for the mobile radio system are usually quite different and incompatible. Integrating such systems would require matching of the numbering schemes.
In FI Patent Application 914655, to solve this problem, a unique subscriber identity code for the fixed transmission network can be derived from the binary subscriber identity code of the CT system by a simple algorithm at the system interfaces or the same CT subscriber identity code can be restored by an inverse algorithm.
In the mobile radio network, the subscriber identification or authentication is performed at least in connection with the registration to the base station and the call setup. Correspondingly, the authentication procedure is usually performed in the CT systems at least in connection with the call setup. A problem with the integration of the CT system and the mobile radio system is that their authentication procedures differ from each other and cannot be altered or interconnected so as to be compatible.